http://www.sciencedirect.com/science/article/pii/S0012821X20300121 Localized deposits of aliphatic organic-rich material have been found on the surface of dwarf planet Ceres, and based on mineralogical context, it has been suggested that these organics formed within Ceres (De Sanctis et al., 2017). However, no obvious source and exposure mechanism can be inferred from the geological settings of the organic-rich terrains. The alternative hypothesis is that the organics were delivered from an exogenic source via impact (Pieters et al., 2018). We numerically simulate the thermal degradation of aliphatic organics delivered to Ceres during hypervelocity impacts, as well as the thermal degradation and excavation of organics from the subsurface. Our models show that exogenic delivery of aliphatic organics is inefficient, as most of the spectral signature of the organic species would be thermally degraded and diluted by mixing with target material in the ejecta blanket of a given crater. Instead, our models suggest that the formation of Ernutet crater excavated a subsurface reservoir of aliphatic organics from a depth of 3-6 km, bringing it close to the surface, where it could be subsequently exhumed by small craters and landslides, compatible with observations.

Boyd, E.S., Amenabar, M.J., Poudel, S., Templeton, A.S., 2020. Bioenergetic constraints on the origin of autotrophic metabolism. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 378, 20190151.

https://doi.org/10.1098/rsta.2019.0151 Autotrophs form the base of all complex food webs and seemingly have done so since early in Earth history. Phylogenetic evidence suggests that early autotrophs were anaerobic, used CO as both an oxidant and carbon 2 source, were dependent on H as an electron donor, and used iron-sulfur proteins (termed ferredoxins) as a 2 primary electron carrier. However, the reduction potential of H is not typically low enough to efficiently reduce 2 ferredoxin. Instead, in modern strictly anaerobic and H -dependent autotrophs, ferredoxin reduction is 2 accomplished using one of several recently evolved enzymatic mechanisms, including electron bifurcating and coupled ion translocating mechanisms. These observations raise the intriguing question of why anaerobic autotrophs adopted ferredoxins as central electron carriers only to have to evolve complex machinery to reduce them. Here, we report calculated reduction potentials for H as a function of observed environmental H 2 2 concentration, pH and temperature. Results suggest that a combination of alkaline pH and high H concentration 2 yield H reduction potentials low enough to efficiently reduce ferredoxins. Hyperalkaline, H rich environments 2 2 have existed in discrete locations throughout Earth history where ultramafic minerals are undergoing hydration through the process of serpentinization. These results suggest that serpentinizing systems, which would have been common on early Earth, naturally produced conditions conducive to the emergence of H -dependent 2 autotrophic life. The primitive process of hydrogenotrophic methanogenesis is used to examine potential changes in methanogenesis and Fd reduction pathways as these organisms diversified away from serpentinizing environments.

Brasseur, B., 2020. A younger 'earliest human migration' to Southeast Asia. Science 367, 147 -148.

http://science.sciencemag.org/content/367/6474/147.abstract The fossiliferous Sangiran dome in Central Java contains the oldest human remains in Southeast Asia and is thus considered to be one of the most important sites in human paleoanthropology. Researchers have discovered 1 more than 100 hominid remains from at least three different early to middle Pleistocene hominid species ( ). Although numerous dating studies have been conducted at this site, the accepted date of earliest hominin et al. 2 migration is controversial. On page 210 of this issue, Matsu'ura ( ) describe their combined use of uranium/lead (U/Pb) dating (crystallization age) and fission-track dating (volcano eruption age) on zircons from three key strata in the hominid-bearing layers of Sangiran. 2 ∼ The Sangiran dome is a recently (Upper Pleistocene) uplifted tectonic dome in a fossil-filled 55-km area with 250-m-thick Quaternary outcroppings. One of the world's first potassium/argon (K/Ar) datings of a hominid- 3 bearing layer occurred on an Australasian tektite found at the site ( ). However, 55 years and 36 absolute datings later, considerable uncertainty remains regarding the potential age of Sangiran's extensive and deep fossiliferous strata. This uncertainty makes it difficult for paleoanthropologists to connect Sangiran hominids to